The field of the disclosure relates generally to actuators and motors and, more particularly, to linear switched capacitance actuators and motors.
Many known motors/actuator devices use magnetic fields as a force transfer mechanism rather than electric fields due to the higher energy densities achieved with magnetic fields using conventional materials and configurations. Such known devises are used extensively for operation of larger devices such as valves and dampers. However, they have some disadvantages for smaller applications, such as operation of robot translatables and aviation devices.
At least some other known motors and actuators use electric fields rather than magnetic fields for electro-mechanical energy transfer. A switched capacitance actuator (SCA) is an electric field-based device that demonstrates an improved energy density over earlier electric field-based devices. The electro-mechanical energy conversion is at least partially a result of the change in the device capacitance with respect to rotor translation. Such SCAs are electrostatic motors that include a translatable portion, e.g., a rotor, and a stationary portion, e.g., a stator, and operate in a manner similar to the magnetic field equivalent of the SCA, a switched reluctance motor (SRM). Both the rotor and stator include multiple electrodes that correspond to magnetic poles in a SRM. When voltage is applied to a stator capacitor electrode pair, a rotor electrode will induce relative motion in the rotor to align with the stator capacitor electrode pair. When the voltage on this stator electrode pair is removed, the appropriate next stator electrode pair that is not aligned with the rotor electrode is energized with a voltage to continue the relative motion.
However, such known SCAs do not match electromagnetic machines with respect to the motion inducing shear stress, i.e., total force or torque output per unit rotor surface area. Therefore, to attempt to achieve parity with electromagnetic devices with respect to power-to-weight ratio, at least some known SCAs compensate for the relatively lower shear stress by increasing the active area of the air gap defined by the SCA rotor and stator. According to Gauss' Law, electric field lines are not required to define closed field loops, and in contrast, magnetic field lines form closed loops that originate and terminate on the magnet. Since the electric field lines do not need to be closed, the rotor surface area may be increased by adding active layers. Another strategy to increase the power-to-weight ratio is to increase the shear stress by improving the dielectric breakdown strength within the gap of the SCA. For example this may be achieved through evacuating the SCA casing. The dielectric breakdown strength of vacuum may be much higher than that of air, which allows the strength of the electric fields in the gap to be larger. However, the evacuation configuration increases the complication of the SCA since the device needs to be securely sealed with a vacuum pump. Another example is that increased dielectric breakdown strength within the gap may be achieved by incorporating inert gases such as sulfur hexafluoride (SF6) and increasing the gas pressure to achieve the desired dielectric properties. However, these configurations also increase the complication of the SCA since sealing is again required. Such configurations are difficult to implement in robotic and aviation applications, at least partially due to size and weight constraints.
Other known SCAs have the gaps filled with a high permittivity, low-viscosity, dielectric fluid. The gap fluid is configured for high frequency wave excitation and the resultant high frequency repetition rates facilitate use of liquids with high dielectric permittivity, i.e., relatively strong dielectric constants (K), e.g., deionized water (with a K of approximately 80). In addition to deionized water, such gap fluids may include, without limitation, vegetable oil (K greater than approximately 3.0), silicone oil (K greater than approximately 2.7), fluorinated oils (K of approximately 1.9), alcohol (K greater than approximately 20) and mineral oils (K of approximately 2.0). The power density of the SCAs is significantly increased if it's electrodes are separated by a high K fluid, which significantly increases the electric force between the electrodes, yet allows for free relative motion of the electrodes. Currently, the existing high K liquids also have high electrical conductivity (S), which renders them unsuitable for SCA applications because as the S increases, electrical losses increase, and machine efficiency decreases. Moreover, if the liquid conductivity is too high, the gap will act as a continuous conductive layer between the rotor and stator, thereby significantly altering the electric field distribution desired for an SCA and, as such, reduces the force and power density. For example, liquids with high K, such as water and alcohol, typically also have a high S, i.e., approximately 5.5*102 micro-siemens per meter (μS/m) and approximately 6.0 μS/m, respectively. The highly insulating liquids with a relatively low S, such as oils as described above (with an electrical conductivity of approximately 12*10−6 μS/m), have a relatively low K.
If two liquids, e.g. a high K/high S fluid and a low k/low S fluid, are mixed and they are miscible, the mixture is uniform at the molecular level and a continuous conductive path across the liquid body is formed due to the universal presence of the high S fluid molecules. As a result, while the K increases moderately, the S, and the associated conduction current, increases rapidly. One effort to produce a high K/low S gap fluid includes using nanoparticle suspension instead of an all-liquid mixture. However, in general it is very difficult to achieve a high loading of nanoparticles, e.g., greater than 10 weight percent, which is needed to achieve a substantially increased dielectric constant without causing mixture stability issues and the associated high S.